A known on-board evaporative emission control system comprises a vapor collection canister that collects fuel vapor emitted from a tank containing volatile liquid fuel for the engine and a CPS valve for periodically purging collected vapor to an intake manifold of the engine. A CPS valve comprises a solenoid that is under the control of a purge control signal generated by a microprocessor-based engine management system. The solenoid acts via an armature that positions a valve element relative to a valve seat to set the extent to which the CPS valve allows vapors to flow to the manifold.
One form of purge control signal is a duty-cycle modulated pulse waveform having a relatively low operating frequency, for example in the 5 Hz to 20 Hz range. The modulation may range from 0% to 100%. This means that for each cycle of the operating frequency, the solenoid is energized for a certain percentage of the time period of the cycle. As this percentage increases, the time for which the solenoid is energized also increases, and therefore so does the purge flow through the valve. Conversely, the purge flow decreases as the percentage decreases.
Changes in intake manifold vacuum that occur during normal operation of a vehicle may also act directly on a CPS valve in a way that upsets an intended control strategy unless provisions, such as a vacuum regulator valve in the purge flow path for example, are included to take their influence into account. When the CPS valve is closed, manifold vacuum at the valve outlet is applied to the portion of the valve element that is closing the opening bounded by the valve seat. Changing manifold vacuum affects certain operational characteristics of such a valve, potentially causing unpredictable flow characteristics.
The particular construction of a solenoid-actuated valve, and certain external influences thereon, may impair certain operational characteristics, such as the start-to-flow point and the incremental low-flow characteristic.
From commonly assigned U.S. Pat. No. 5,413,082, inter alia, it is known to incorporate a sonic nozzle function in a CPS valve to reduce the extent to which changing manifold vacuum influences flow through the valve during canister purging. From U.S. Pat. No. 5,373,822, it is known to provide pressure- or force-balancing of the armature/valve element.
From other patents, such as commonly assigned U.S. Pat. No. 4,901,974 issued Feb 20, 1990, it is known to incorporate noise-attenuating bumpers to absorb impact forces created by abutment of the armature with stops as the armature reciprocates.
Each of U.S. Pat. No. and U.S. application Ser. No. 08/918,071 discloses a canister purge solenoid (CPS) valve that is effective over a wide range of intake manifold vacuum levels to consistently cause the actual purge flow to more predictably equate to that intended by the purge control signal irrespective of changing intake manifold vacuum. That CPS valve integrates force-balancing and intake manifold vacuum de-sensitizing so that the start-to-flow duty cycle is significantly de-sensitized to changing intake manifold vacuum. It includes a sonic nozzle structure at its outlet. The valve exhibits quite consistent opening as its valve element unseats from the valve seat; it also exhibits quite consistent closing as the valve element reseats on the valve seat. Because of these consistencies, which are relatively quite well-defined and predictable, the duration within each duty cycle for which the sonic nozzle structure at the valve outlet functions as a true sonic nozzle is also quite well-defined and predictable, being equal to the duration of the duty cycle less the durations of valve element travel at initial valve unseating and at final valve re-seating where the proximity of the valve element to the valve seat prevents the sonic nozzle structure from operating as a true sonic nozzle, uninfluenced by the extent of flow restriction present between the unseated valve element and the valve seat. The sonic nozzle structure will therefore function as a true sonic nozzle over an entire duty cycle except for these initial unseating and final re-seating transitions. By making the valve element travel during which these transitions occur relatively short, the sonic nozzle structure can function as a true sonic nozzle over a larger portion of a duty cycle. Therefore, actual mass purge flow that will occur during a duty cycle may be accurately correlated to the purge control duty cycle signal, and hence made well-defined and well-predictable.
Because of the improvements provided by these valves, it is believed that certain particulate material entrained with the purge flow may create noticeable effects on valve performance. Particulate material may hang up within the valve, and if this occurs proximate the valve seat, the ability to properly seal the valve assembly to the seat may be at issue. Accordingly, it is believed desirable to provide a solution for such contingency, especially because a purge valve is not typically disassembled for periodic maintenance service during its life in a motor vehicle.